1. Field of the Invention
The present invention relates generally to ion implantation and, more particularly, to a method and apparatus for the ion implantation of spherical surfaces.
2. The Prior Art
Ion implantation is by now a well known method that was originally developed in the semiconductor industry to effect precise doping of silicon wafers with impurities. From there, the method spread to the ion implantation of metals and compounds, see Treatise on Materials Science and Technology, Vol. 18, "Ion Implantation," 1980, Academic Press, Inc. See also a recently issued U.S. Pat. No. 4,693,760, assigned to the common assignee herein, the Spire Corporation, and entitled "Ion Implantation of Titanium Workpieces Without Surface Discoloration." See further an article authored by M. S. Dresselhaus et al of M.I.T. "Ion Implantation of Polymers," Mat. Res. Soc. Symp. Proc., Vol. 27 ( 1984), pp. 413-422, and another by G.K. Wolf, "Ion Bombardment Chemistry," Nucl. Instrum. Methods 139 (1976) 147. See also "Ion Beam Modification of Materials for Industry," Thin Solid Films, 118 (1984) 61-71; "The Wear Behavior or Nitrogen-Implanted Metals," Metallurgical Transactions, A 15 (1984), 2221-2229; and "Wear improvement of surgical titanium alloys by ion implantation; J. Vac. Sci. Tech. A3 (6) Nov./Dec. 1985, 2670-2674.
Some of the above mentioned work also involved the ion implantation of spherical surfaces as, for instance, the ball part 26 of a hip prosthesis 12 in the said U.S. Pat. No. 4,693,760. Inasmuch ion implantation is a line-of-sight process, spherical surfaces represent a geometry that is particularly difficult to ion implant, especially ion implant uniformly about its spherical surface. The uniform ion implantation of spherical surfaces, in particular when needing to process large quantities thereof, has thus presented special problems. The problem has been aggravated even more when wanting to ion implant spherical surfaces of different sizes and of different materials.